The oft-injured Severino was placed on the 15-day IL on Thursday, one day after leaving his start against Cincinnati with right shoulder tightness.
Severino had an MRI that showed a low-grade strain to his right latissimus dorsi muscle. The team did not yet have a timetable for his return, manager Aaron Boone said.
"I'm optimistic on it,'' manager Aaron Boone said.
It's the first significant setback this year for a terrific New York rotation that's remained almost entirely healthy, a huge reason the Yankees have built the best record in baseball with a few days left before the All-Star break.
"I don't think it's (anything) bad,'' Severino said.
New York selected right-hander Ryan Weber to the major league roster from Triple-A Scranton/Wilkes-Barre.
The team also reinstated reliever Jonathan Loaisiga from the 15-day injured list. The right-hander had been sidelined since May 23 with shoulder inflammation.
"He's in a really good place mentally. He's ready to go," Boone said. "He feels absolutely ready to roll, and excited to get him back tonight."
Severino is 5-3 with a 3.45 ERA and 95 strikeouts in 86 innings. He's made 16 starts after missing most of the past three seasons due to injuries.
He went 19-8 with a 3.39 ERA in 2018 and was selected for his second consecutive All-Star team. After signing a $40 million, four-year contract in February 2019, Severino strained his latissimus dorsi muscle and didn't make his first start that year until Sept. 17. He made two more in the postseason, then had Tommy John surgery on Feb. 27, 2020.
Severino returned last Sept. 21 and made four relief appearances plus another in the AL wild-card game.
"He knows how important he is to this team. We all know what he's been through the last few years," Boone said.
After Boone and an athletic trainer came to the mound, Severino finished the inning. He went to the mound to start the third and threw a warm-up pitch, then walked off and was replaced by rookie JP Sears, who was optioned back to Triple-A after the game.
Severino averaged 94.1 mph with 18 fastballs, down from his season average of 96.1 mph.
"He obviously knows his body well and obviously with the injury history he's had over the last few years and what he's had to deal with, we weren't going to mess with it. And he was clear, like, "'I'm not going to do anything stupid here,'" Boone said.
"It seemed like at the end of the second inning, kind of stuff-wise came back a little. Got the punch-out, got the top of the order out. And it looked like, OK. But then when he went out to warm up that next inning, just felt that tightness there still. And just, I think smartly pulled himself."